Tuesday, July 7, 2009
Fixing Flutter Is Nothing New
Investigating violent oscillations that led to structural failure
MAINTAINING STRUCTURAL INTEGRITY IN THE AIR. Within a six-month period, two Lockheed Electra airliners (one operated by Braniff Airways) were felled due to a phenomenon investigators termed “propeller-whirl flutter,” which could tear an aircraft apart in 30 seconds.
The NTSB noted that the CH 601XL relies on control-cable tensioning to add stiffness to the aileron system, provide a higher natural-resonant frequency of the aileron/wing combination and raise the airspeed at which flutter may occur. Aircraft in the normal, utility, aerobatic and commuter categories (which are FAA-certified, unlike the Zodiac) typically use a combination of structural stiffness and flight controls that are balanced through the use of added mass (typically a weight) for protection against flutter. The NTSB says this provides more direct protection against flutter; it notes that some LSA use mass-balanced flight controls.
Government certification is no guarantee that flutter won’t be a problem. Over the years, the NTSB has reviewed flutter-related accidents involving familiar makes and models. The original C-141 military transport had early issues with severe tail flutter, as did the F-15 jet fighter. F-16s with wing pods mounted in certain positions also experienced wing flutter.
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